Why my sphere is compress at the end
It should be that you scaled the curve after setting the constraint.

select the curve and hit CTRLA and apply scale...

it should be fixed. Screenshots will follow if needed...

The answer is already accepted, but since I already have done screenshots, here they are, showing the problem and the solution... may help others understand.

first you set the follow path animation:



then you Z-scale  the path and happens this



but you can select the path, and apply the Z-scale



so you get the correct ball shape, no more deformed...



edit

screenshots showing curve frames count for the same curve Z-scaled (applied)

72 frames for both curves:



the second curve has frames doubled now...